Metal-impregnated heating rods for electric



Feb. 21, 1956 F. J. MANN 2,735,831

METAL-IMPREGNATED HEATING RODS FOR ELECTRIC FURNACES AND METHOD OF MAKING SAME Filed Dec. 27, 1952 FIG? INVENTOR RANZ Jose H MANN ATTORNEY METAL-IMPREGNATED HEATING RODS FOR 7 ELECTRIC FURNACES AND METHOD OF MAKING SAME Franz Josef Mann, Erlangen, Germany, assignor to Siemens-Planiawerke Aktiengesellschaft fiir Kohlefabriirate, Metingen bei Augsburg, Germany, a corporation of Germany Application December 27, 1952, Serial No. 328,187 Claims priority, application Germany January 25, 1952 16 Claims. (Cl. 13-25) This invention relates to resistor rods provided with enlarged terminal ends, for use in electric furnaces having relatively thick walls, and is particularly concerned with resistor rods having formed porous bodies preferably made of silicon carbide or of metallic powder or a powder mixture or metal compositions, alloys, or heatresisting oxides.

.In order to keep the temperature at the terminal ends of the resistor rods as low as possible, such terminal ends were in prior structures carried to the outside of the furnace walls, so as to provide for effective cooling. However, the furnaces must be equipped with relatively thick walls so as to keep thermal losses low; and, if the contact with the resistor rods is to be made outside of the furnace walls, they must be provided with very long terminal ends, resulting in increased cost and insufiicient utilization of the portions thereof which extend through the furnace walls. It has therefore been proposed to keep the terminal ends of the resistor rods relatively short, as compared with the wall thickness of the furnace, and to dispose in the furnace walls the contact members or heads for engagement with the associated terminal ends of the resistor rods. Resistor rods are used having thickened terminal ends which have, due to their greater crosssectional areas, a considerably lower resistance than the heating portions thereof. The contacting surfaces of the resistor rods were provided with a conductive metallic coating in order to keep the transition resistance at the contacting surfaces between the contact members and the associated terminal ends of the resistor rods as small as possible.

One object of the invention is to reduce the crosssectional areas of the terminal ends of the resistor rods so as to increase the heat capacity of the corresponding furnace by providing an increased number of resistor rods closely adjacent one another.

The thickened terminal ends of the resistor rods are, in accordance with the invention, impregnated with metal that flows in melted condition so as to obtain, with reduced cross-section, the necessary electrical conductivity and to keep low the temperature produced by the current flow in such terminal ends. The metal penetrates deep into the pores of the portion of the resistor rods that is to be impregnated and fills the hollow spaces throughout the entire cross-sectional area thereof. However, this metallic impregnation can be accomplished only if the corresponding formed body exhibits the required porosity, as is the case, e. g., with formed bodies of silicon carbide.

Suitably formed bodies having the desired porosity may also be produced metallurgically from powders of high-melting metals and heat-resisting oxides. Among such metals is molybdenum intermixed with a nonconductive material such as zirconium oxide or other oxides of the alkaline earth metals.

The use of porous formed resistor rod bodies permits, as an advantageous feature, to provide upon the heating nited States Patent ice portion thereof a thin protective coating which is insensitive to high temperatures and protects it against chemical attacks of the hot furnace gases.

Such a protective coating may be formed from initial substances which are in melted condition or in solution adapted to penetrate into the pores of the rod and which may by subsequent thermal or chemical treatment be transformed to compositions yielding a dense, chemically inert but heat-resisting coating. Aluminum oxide spinell may be mentioned as a suitable composition of this kind that may be formed by the protective coating.

The radial surfaces of the terminal ends of the resistor rods facing the heating compartment of the furnace are suitably also provided with a protective coating such as is formed on the heating portion thereof.

As already mentioned, the formed body of the resistor rod must have a certain porosity as a requirement for the metallic impregnation of the terminal ends as well as for the anchoring of the protective layer or coating on the surface of the heating portion thereof. The resistor rod is for this purpose made as a unitary and integral body having the terminal ends and also the heating portion formed of the same raw material and in the same production process.

It would appear feasible in the case of resistor rods made metallurgically of powders comprising metals and oxides to enrich the terminal portions metallically from the start. However, since the corresponding formed bodies are made by molding or pressing together the various powders prior to the sintering, there will result difliculties in introducing different powder mixtures for the different portions of the bodies into the corresponding mold.

In accordance with a further feature of the invention, the heating portion and also the terminal portions of the resistor rods are made of the identical basic or raw material having a porosity which permits impregnation with metals that flow in melted condition, and which also permits penetration of certain initial substances to form the protective coating.

An example of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows an embodiment of the new resistor rod in part sectional and part elevational view; and

Fig. 2 indicates a portion of a furnace wall in section, also showing in elevation the terminal portion of a resistor rod, and in sectional view the contact head or member associated therewith.

The new resistor rod, as shown in Fig. 1, comprises a formed silicon carbide body having thickened terminal 7 portions 1 at the opposite ends of a heating portion .2.

The axially outwardly facing walls of the terminal portions 1 are cone-shaped so as to increase the contact surfaces formed thereby. These terminal portions 1 are impregnated with metallic silicon throughout their entire cross-sectional areas. The silicon is introduced in molten flowing condition by the application of pressure so as to penetrate into the pores of the terminal portions.

The heating portion 2 is provided with a protective layer of aluminum oxide spinell which is anchored in the pores. This protective layer or coating 3 also extends over the radially extending inner walls of the terminal portions 1 which face the heating compartment of the furnace.

In Fig. 2 numeral 4 indicates part of the furnace wall. Current is conducted to the resistor rod 1/2 over the contact head or member 5 made of graphite or suitable metal and provided with a duct 6 for the circulation of a cooling fluid. Numeral 7 indicates the current-supply terminal which is secured on the head 5 by a clamp 8.

A spring (not shown) is provided which extends from the furnace wall to press the contact head 5' against the associated terminal portion ofj'the resistor rod. The contact head 5. and the terminal portionl of the ,I'CSISIOI rod are disposed inside of "a tubular insulatingmember 9? which is lined'with an asbestos tubing .10.

The opposite right hand'terminal portion .of the heating rod (not shown in Fig 2) is secured and is' supplied with current by means similar. to those shown in con: nection with the illustrated'left hand "portion thereof:

Ilclairnz.

1." A resistor rodfor. an electric furnace comprising an elongated body made of porous material and forming'a centrally axiall'y extending rodlike heating portion, and an enlargedite'rminal portionat each end of 'said heating portion which is integral therewith, said enlarged terminal portions being impregnated with metalithrough:

out their respective,crosssectional areas and havingdue to such' impregnation a specific resistance which islower than that of said heating portion.

2. The heating rodas defined in claim 1, having a body made of silicon carbide.

3,. Theheatingrod as defined in-claim l, havinga, body made of powdered material comprising metal and metal oxide components.

4. The. heating rod as defined in claim 2, comprising.

a protective coating carried by said heating portion.

5. The heating rod as definedin claim 2, wherein said terminal portions are'irnpregnated with metallic silicon. 6. Theheating red as defined in claim 1, wherein the axially outwardly facing ends 'ofsaid enlarged terminal portions are cone-shaped.

7'5 The heating rodas definedin claim 4,'comprising aprotective coating'carried by theaxially inwardly fac-- ing' walls of said enlarged terminal portions.

8; The method of making a resistor rod for an electric furnace having an elongated body exhibitingacentrally axially extendingrodlike heating portion and an enlarged terminalrportion at each endof said'heating portion, said', method comprising the following steps, namely," taking a raw material and shaping it to form said 'body integrally therefrom, said integral body being a' porous body, and impregnating said enlarged terminal portions with .metal throughout the entire, crosssectional areasthereof, said enlarged terminal portions.

11. The method defined in claim 10, wherein said raw material -is"-silicon-carbide "and-wherein said metal impregnation is metallic silicongrand wherein said protective coating is formed by aluminum oxide spinell.

12. In the art of producing for use in electric furnaces resistor rods. comprising an elongatedbody having a centrally axially extending rodlike heating portion and an enlarged terminal portion at each end of said heating portion, the method of making such resistor rod comprising the following steps, namely, taking .raw material and forming said body integrally therefrom, said integral body exhibiting aporosity adapting it for impregnation by a molten metallic material, impregnating said enlarged terminal portions with said 'metallic material throughout their entire cross-sectional areas by the application of pressure, and applying to said-heating portion and to the axially inwardly facing; walls of said enlarged terminal portions a material to form a protective coating thereon- 13? The method I defined in claim 12, wherein said raw material is silicon carbide.

14.: Themethod defined in claim 12," wherein said raw material is a powder comprising metal and metal oxide components.

15. The method defined in claim 12, wherein said raw material is molybdenum intermixed LWl Lh nonconductive-oxide: a

16; In an electric resistance-furnace having a wall forming aheat-ing chamber, a resistor body comprising an elongated 'rodlike heating portion and an enlarged terminal portion" at oneend thereof and integral therewith, said'terminal portion-beingimpregnated with metal throughout its respective cross-sectional area and having due-to such impregnation --a-specific' resistance which is lowerthanthat-of 'said heating p ortion,asaid heating portion and th'eaxially inwardly directed radially extending wall of 'said enlarged tenninal portion adjacent said heating portion carrying a' material forming a protective coating thereon, a contact member disposed in axial abutment=with the axially outwardly facing end of said enla-rgedv terminal portion; and a tubular member of a length corresponding substantially at least to the thickness of said furnace Wall forembracing said enlarged terminal portion and adjacent contact member to mount such portionand member in saidfurnace wall;

PreterenceseCitedginzthe file of this .patent UNITED STATES PATENTS 1,553,379 Hellrnund Sept. 15, 1925 FOREIGN PATENTS 367,403" Great Britain Feb. 18, 1932 409,031 Great Britain Apr. 3, 1951 977,650 France Apr. 3, 1951 

1. A RESISTOR ROD FOR AN ELECTRIC FURNACE COMPRISING AN ELONGATED BODY MADE OF POROUS MATERIAL AND FORMING A CENTRALLY AXIALLY EXTENDING RODLIKE HEATING PORTION AND AN ENLARGED TERMINAL PORTION AT EACH END OF SAID HEATING PORTION WHICH IS INTEGRAL THEREWITH, SAID ENLARGED TERMINAL PORTIONS BEING IMPREGNATED WITH METAL THROUGHOUT THEIR RESPECTIVE CROSS-SECTIONAL AREAS AND HAVING DUE 